Here is what we did: After agreeing to sex twice per week and then having H's definition of a "week" set me into unnecessary rages (meaning, he'd forget that we hadn't done it that week and I'd go ballistic), we then came up with the following agreement:
We never go more than 3 days without having sex.
This seemed to keep it fresh in his mind...as an activity that we do FREQUENTLY and not something that you just put out of your mind until the urge hits, or loneliness hits or whatever. It kept me from turning into a bitch and it jump-started his own desire.
